Since a medical laser imager is required to express a diagnostic image with tone reproduction, there is a strong demand for a basic function to output densities of the diagnostic image stably at any time. Further, there is a feature that the medical laser imager is used in many cases in such a way that a patient is continuously radiographed on several sheets or around four sheets as the maximum for the front and the sides of the chest or the front, the sides, the side anteflexion and the retorflexion when photographing and/or printing the lumbar vertebra.
These medical laser imagers are equipped with a so-called calibration function to control an image forming section in such a way that a digital or video signal (a designated density signal) is expressed with a predetermined density on a film.
Immediately after the calibration, the predetermined density is retained for a while, however, it fluctuates due to several factors as time passes after the calibration. Especially, it is well known that densities reproduced by a heat developing process fluctuate easily.
For example, there are the following factors for the fluctuation:
(1) Exposure system fluctuation due to an environmental temperature,
(2) Characteristic fluctuation of heat developing process associated with film development,
(3) Sensitivity fluctuation of film stored in a machine,
(4) Characteristic changes of a heat developing drum,
(5) Film which has different heat developing characteristics.
In the above fluctuation factors, since a degree of an influence on a density fluctuation due to the fluctuations described in factors (1) and (2) can be predicted to a certain extent by a monitor for the inside temperature of the machine, the finished density can be corrected so as to keep the predetermined density by a feed-forward control (FF). On the other hand, as it may be difficult to predict fluctuations described in factors (3)–(5), a so-called patch density feed-back method (FB) is used for these fluctuations in such a way that a finished patch density influenced with overall fluctuations including the fluctuations described in factors (3)–(5) is measured, and then a feed back (FB) correction is applied the following print based on the measured patch density.
In this patch density feed-back method, an about 5×10 mm rectangular area at a predetermined position on a film is exposed with a predetermined amount of light, the finished density of the area is measured, and then an exposure amount and/or a heat developing condition are adjusted so as to make the following print to have an optimum density on a basis of a difference between the measured density and a fundamentally-obtainable density (hereinafter, referred as a reference density).
Consequently, when the reference density is set incorrectly, even though a developing system reproduces proper images (proper density), an adjustment system judges that it is not proper and a process system changes a processing condition, as a result, density is lowered or increased.
It is undesirable to set a reference density at a constant value since an exposure system and a heat developing process include dispersion factors in an each individual apparatus.
There are a lot of cases that a combination of a calibration function and a patch density control is used in a medical laser imager to stabilize gradation and attain an absolute density.
Incidentally, since a surface-type heater is attached on an inner peripheral surface of a cylindrical aluminum element tube in a heat developing drum, a joint seam of a heating element (a portion where heating is not applied) is inevitably formed due to a manufacturing process and a temperature un-uniform portion inevitably exists in an circumferential direction. Depending on the situations, a joint seam takes place along a longitudinal direction on a heated drum. In order to make the drum surface temperature corresponding to the point where a heater is not provided, to be the same temperature as that of other places as far as possible, the heater attaching density (Watt distributing density) in the vicinity of the joint seam of the heater is set at relatively higher level on comparison with other places. With the heater control, it is necessary to compensate the quantity of heat in the amount taken away from the drum surface by a film when the film is conveyed and processed. Consequently, in general, a sensor is provided on the internal or external surface of the drum and the heater is controlled with ON/OFF control according to sensor output.
This ON/OFF control is activated even in the standby mode, waiting for film processing within which a drum temperature stays within a certain temperature fluctuation (hunting width is small even though there is hunting, for example, the drum temperature stays within 1° C. so that the influence on density is small. However, as the film processing continues, this hunting width becomes wider (more than 1.5° C.) and the influence on density becomes noticeable.
Furthermore, overshooting or undershooting appears based on timing of heater control when film processing is finished. For example, when a heater is turned on at the time when film processing is stopped, overshooting of the drum temperature results since control is in a mode, which supplies heat for continuous processing of film. Executing calibration in this situation results in abnormal density on the wedge-pattern portion developed by a drum surface corresponding to a seam whose temperature was abnormal. This abnormal temperature tends to appear on an area where heaters were superposed to avoid a joint seam between heated portions. When subsequently printing a diagnostic image by using a lookup table for calibration, appropriate direct tone-reproduction by density cannot be obtained since the lookup table was produced based on measurements of this abnormal density to cancel abnormal density itself, and adversely influences diagnostics.